How FrostDateCheck Calculates Your ZIP Code's Frost Dates

Two datasets, joined per ZIP

Hardiness zone comes from the USDA Plant Hardiness Zone Map, built from 30 years of PRISM climate data. Frost dates come separately, from NOAA's 1991-2020 climate normals: the last-spring-freeze and first-fall-freeze probability elements at 32°F, reported by individual weather stations, not by ZIP code. Every ZIP in this dataset is joined to its nearest reporting station by straight-line distance.

Why some ZIPs have no frost figure

About 1 in 40 ZIP codes in this dataset has no usable last-frost or first-frost figure, because the nearest station that reports a hardiness zone does not separately report the frost-probability elements. Those pages say so directly rather than substituting a neighboring station's numbers silently. The hardiness zone, which comes from a denser climate grid rather than individual stations, is still shown.

Three dates, not one, per event

NOAA publishes each freeze event as a probability curve, not a single day. This site surfaces the median, the point half of the last 30 years fell before and half after, plus the early and late ends of that curve. The range guide explains what each one means for a planting decision, and why the labels on NOAA's own source columns read in a way that is easy to misread going into the last-frost date specifically.

Growing season, and the ZIP codes with none

Growing season length is a separate NOAA element (median frost-free days) rather than the gap between this site's own median dates, so it can be present even on the small number of pages with no last- or first-frost figure. It is used to rank every ZIP nationally and against its own state.

Where to go next

See the range this plays out across: Stanley, ID, Farber, MO, Indian Creek, FL.

Hardiness Zone vs. Frost Date: Different Questions · What the Early, Typical and Late Frost Dates Actually Mean